The present invention relates generally to electrical power instrumentation and more particularly relates to a watt/watthour transducer for metering the rate and quantity of electrical energy transmission.
The conventional watt/watthour metering apparatus which is related to the present invention typically has voltage and current input scaling means for providing a signal proportional to power line voltage and a signal proportional to power line current. These signals are applied to a multiplier circuit which provides an output signal having a value substantially proportional to the power or rate or energy flow through the power line.
Some systems, such as that shown in U.S. Pat. No. 3,794,917, have a multiplier which includes a pulse width modulating circuit. In such a circuit, one of the input signals or a signal proportional thereto is modulated by the other input signal or a signal proportional thereto. The output of such a pulse width modulator arrangement consists of a series of pulses having a height proportional to the instantaneous value of either the current or voltage in the power line and having a pulse width proportional to the instantaneous value of the other.
The average value of this series of pulses is proportional to the instantaneous power of the power line. Consequently, the output of the modulator may be filtered and amplified to provide an output signal indicative of instantaneous power (actually power averaged over a very small time interval dependent upon the response time of the system which is typically less than a few seconds).
In addition, a signal proportional to the modulator output signal or the modulated output signal itself may be integrated with respect to time to provide a signal corresponding to total watthours of energy through the power line over a given time interval. The present invention relates to improvements in watt/watthour metering apparatus.
It is conventional in a circuit of the above type to utilize voltage signals and voltage devices. This means that the information contained at various points in the circuitry is related to the voltage between particular nodes rather than to the current flow. For example, the modulators of the prior art multiplier circuits ordinarily modulate the voltage related signal with the current related signal being the modulating signal. This is done because it is the current axis which must be maintained very linear in the circuitry. On most modulator circuits the modulating axis is more linear than the modulated signal axis. So most circuits modulate the voltage related signal with the current related signal. This provides an output voltage having an average value approximately equal to the instantaneous power of the power line.
This average voltage is then conventionally filtered and amplified by a voltage amplifier and subsequently converted to a current signal by a high output impedance amplifier to provide a current source output to the watt output load, such as a recording device. Even in a circuit providing a modulator output current proportional to instantaneous power, the current signal is conventionally converted to a voltage signal by sinking it through a resistance and then this voltage signal is amplified.
It is typical in the integrating section of the watt/watthour apparatus to provide circuitry which not only integrates the watt related signal with respect of time but quantizes the signal into a series of countable output pulses each pulse indicating a unit of energy. Such pulses may conveniently be counted or accumulated in a suitable register or counting device to indicate the total energy through the power line.
One problem with such conventional circuits which depend upon one or more voltage signals is that the offset voltages of typical op-amps drift and the voltage drops across various circuit elements such as the modulator cause considerable error. This error is a particular problem in equipment for which error tolerances must be kept within a percentage of reading rather than within a percentage of full scale because a small offset error may be a major portion of small reading. In fact, because of offset error one leader in the field has declared a charge compensation integrator to be unsuitable for watthour circuitry.
It is therefore an object of the invention to eliminate such voltage related error and to greatly improve the percent of reading accuracy of a watt/watthour transducer.
It is a further object of the invention to reduce the cost of a watt/watthour transducer by eliminating some components conventionally required and by minimizing the number of elements while at the same time increasing the accuracy of such circuitry.
In conventional circuits the modulator output signal which is proportional to instantaneous power is filtered and amplifier by a series of cascaded stages. Ordinarily, these consist of a filter stage followed by a voltage amplifier. Thus, in a typical design each cascaded stage performs its own particular function. To applicant's knowledge active filters have not been used in watt/watthour meters. Active filters are generally regarded as voltage input voltage output devices.
The pulse width modulator of a conventional multiplier circuit ordinarily has a voltage output. The voltage output makes it quite easy to send the voltage signal both to the watt output and to the watthour output sections. However, because of the errors caused by having voltage signals in a watt/watthour transducer, it is necessary for purposes of the present invention to have a modulator which can provide two current outputs both of which can be sunk to ground and which are absolutely identical so that one may be sent to the watt output section and one may be sent to the integrator of the watthour section.
It is therefore an object of the present invention to provide a pulse width modulator circuit having two current outputs both floating with respect to ground so that they can be sunk to ground in two different circuits and always of identical magnitude.
It is a further object of the invention to provide such a modulator exhibiting the high output impedance characteristic of current sources.
Further objects and features of the invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings illustrating the preferred embodiments of the invention.